1. Field of the Invention
The present invention relates in general to oxygen sensors and more particularly to a ceramic heater for use in an oxygen sensor of the type having an oxygen ion conductive tube of solid electrolyte. The present invention further relates to a method of producing a ceramic heater for such an oxygen sensor.
2. Description of the Prior Art
An example of a ceramic heater for use in an oxygen sensor of the type described above is disclosed in Japanese Patent Provisional Publication No. 59-91357. The ceramic heater is an alumina ceramic heater and in the form of a round, straight bar correspondingly to the inner circumferential surface of the solid electrolyte tube.
Advantages of the ceramic heater in the form of a round, straight bar are that it has a heat generating surface in the vicinity of the inner circumferential surface of the oxygen ion conductive tube of solid electrolyte and capable of heating the solid electrolyte tube substantially uniformly, that its transverse rupture strength is substantially uniform with respect to any transverse direction, and that it is shaped so as to have a less possibility of causing a concentrated thermal stress.
However, as shown in FIGS. 9A and 9B, the ceramic heater of this kind is produced by preparing a prefired, hollow cylindrical ceramic core 201, placing around the ceramic core 201 a ceramic green sheet 202 on which a high melting point metallic thick-film pattern 203 for a heat generating resistor is previously printed, then bonding the green sheet 202 to the ceramic tube 201 by application of force, and cofiring the ceramic tube 201 and the green sheet 202 and allowing them to be turned into a ceramic heater 200. A disadvantage of the prior art bar ceramic heater is a high cost since the ceramic core requires a prefiring process, the process for winding the ceramic green sheet on the ceramic core requires a number of steps or particular and expensive devices, and a process for bonding the ceramic green sheet to the ceramic core by application Of force requires an expensive machine such as a rubber press. Another disadvantage is that it is difficult to carry out the above described winding and bonding of the ceramic green sheet precisely as desired and therefore peeling off of the ceramic sheet wound on the core may possibly be caused.